Field of the Disclosure
Embodiments disclosed herein relate generally to riser connections for conduits used in a marine drilling riser. More specifically, embodiments disclosed herein relate to a riser connection and method for joining together sections of marine drilling riser using removable shear elements.
Background Art
A marine drilling riser (“riser”) is a jointed conduit which extends generally from a subsea blowout preventer (“BOP”) stack located at the seabed to a floating drilling vessel such as a drillship or semisubmersible. Riser joints are typically between about 50 and 80 feet long and may be connected together by any of various forms of riser connectors to form a riser string. The riser functions to extend the wellbore from the seafloor to the surface and is used to return drilling fluid (or “drilling mud”) and wellbore cuttings from a wellbore drilled into the seabed beneath the subsea BOP stack to the floating rig. In addition, a typical riser may include a number of auxiliary conduits positioned externally to the main riser conduit, which may include choke and kill lines, a riser mud-boost line, at least one hydraulic power conduit, and at least one subsea BOP control umbilical. Typically, the drilling riser joints may also be fitted with jacket-like foam buoyancy segments designed to reduce the apparent weight of the riser joint in seawater.
Riser connectors must withstand high, fluctuating loads over a very long service life (up to 20 years), while remaining robust, inexpensive to manufacture and repair, and as light as possible, consistent with their intended service. In modern offshore drilling, it has also become increasingly beneficial that the riser connector can be assembled and disassembled quickly in order to minimize riser running time, especially because drilling is now taking place in deeper water, and because the cost per day for floating drilling vessels are very high.
For example, the most common type of riser connector in use today (e.g., a bolted flanged connector) can be assembled or disassembled at a rate of only about 3 to 4 joints per hour. For modern riser joints, typically about 75 feet in length, this yields a tripping rate on the order of 300 feet per hour. Tripping-out a 6000 foot long riser would therefore take approximately 20 hours. Because modern floating drilling vessel costs may exceed $20,000 per hour, costs may be minimized by reducing the riser trip time as much as possible. Despite the fact that they are very slow to run, bolted flanged riser connectors do, however, offer the advantages that they are reliable, repairable, and relatively inexpensive.
Various attempts have been made in the past to produce a riser connector which may be run more quickly than a conventional flanged marine drilling riser connection, while retaining its advantages. Some prior-art marine riser connectors use a threaded connector; however, because the riser must carry auxiliary conduits, such as the choke and kill lines, as well as buoyancy segments, a threaded riser connection for a marine drilling riser must typically either be of the “union” type, such as taught in U.S. Pat. No. 4,496,173 (“the '173 patent”) issued to Roche, or must include a provision for the central riser tube to otherwise rotate relative to the auxiliary conduits and buoyancy segments, as taught in U.S. Pat. No. 6,419,277 (“the '277 patent”) issued to Reynolds.
FIG. 1A illustrates the prior-art threaded marine drilling riser connection taught in the '173 patent. The riser connection includes an upper riser pipe section 1, a lower riser pipe section 2, and choke and kill line sections 3A, 3B, 4A and 4B that are supported by upper riser pipe section 1 and lower riser pipe section 2 respectively. The choke and kill lines are joined together at the riser connector by choke and kill connectors 5A and 5B. Female union box member 6 on upper riser pipe section 1 is threaded by spin-up threads 6A to pin member 7, which is welded to lower riser pipe section 2. Preload is applied to the riser connection by power threads 8A between female union box member 6 and power ring 8. Spin-up threads 6A are opposite hand to power threads 8A, that is, one is right-handed and the other is left-handed.
In order to prevent female union box member 6 from becoming loosened because of vibrations from drilling and the action of subsurface currents on the riser string (such as vortex-induced vibration, or VIV), a locking member or key 9 is slidingly displaced into notch 10 to lock the female union box member 6 against rotation relative to pin member 7.
FIG. 1B illustrates the prior-art threaded riser connection taught in the '277 patent. Riser joint 15 has male (“pin”) end threads 11 and female (“box”) end threads 12. Auxiliary conduits 13, such as choke & kill lines, mud boost lines, and hydraulic conduits, have auxiliary line connectors 17, and are attached to marine drilling riser joint 15 by means of flanges 14 and bearings 16. Because flanges 14 are coupled to joint 15 through bearings 16, joint 15 can be rotated while flanges 14 and auxiliary conduits 13 remain rotationally fixed. This enables joint 15 to be connectable to other such joints using conventional threaded coupling methods.
Due to the requirement for a large diameter bearing, which must survive relatively high cyclical loads in a salt-water environment, and due to the difficulty of applying high make-up and break-out torques in and among the auxiliary conduits, the riser connectors of the '277 patent may be prohibitively expensive to build and use.
Either of these riser connection types may be expensive to manufacture, and may rely on the application of very high make-up torque to achieve sufficient axial preload. Further, some provision must generally be made to insure that the threads are locked in a made-up position so that they don't unscrew or “back-out” due to, for example, cyclic loads or vibration, in particular vortex-induced-vibration (“VIV”). In addition, threaded connections are generally not designed to share the loads evenly and efficiently along the axial length of the threads, and are typically subject to the same fatigue limitations as any shouldered threaded connection. Finally, threaded riser connections installed on the riser are difficult, if not impossible, to repair, and in no known example are they repairable on-board a typical floating drilling vessel.
Other prior-art riser connections use a “breech-lock” or “bayonet” or interrupted-thread arrangement, such as taught in U.S. Pat. No. 4,097,069 (“the '069 patent”) issued to Morrill and U.S. Pat. No. 4,280,719 (“the '719 patent”) issued to Daniel. Such “breech-lock” connectors typically make-up or break-out in less than one revolution, are very robust, and typically may be tripped very quickly. However, they generally still require a very high make-up torque and some mechanism to prevent accidental break-out, and are very heavy and extremely expensive to build. In particular, because the load-bearing part of a breech-lock style connector must necessarily extend to only about half of the circumference of the connector, axial loads are carried by the connector in a discontinuous fashion, and the load-bearing parts must therefore be extremely robust, which consequently makes them very heavy and expensive.
FIG. 2A illustrates the prior-art “bayonet” or “breech-lock” type riser connection for a riser as taught in the '069 patent. Riser joint 23 has female connector member 26, male connector member, and auxiliary line flanges 22 attached, as by welding. Auxiliary line flanges 22 support choke auxiliary line 20 and kill auxiliary line 21, which have choke line connector 20A and kill line connector 21A respectively. Female connector member has shoulder 26A which supports connector nut 24. Connector nut 24 has locking mechanism 29 and female tapered lugs 25 with upper inclined surfaces 25A. Male connector 27 has male tapered lugs 28 with lower inclined surfaces 28A.
When making up the riser connection of the '069 patent, the connector nut 24 is lowered over male connector 27, and the connector nut is rotated such that the lower inclined surfaces 28A on male tapered lugs 28 engage with the corresponding upper inclined surfaces 25A on female tapered lugs 25. Finally, locking mechanism 29 is engaged to insure against loosening (or “back-out”) of the made-up riser connection.
FIG. 2B illustrates a prior art double-row “bayonet” type riser connection for a riser, as taught in the '719 patent. Riser joint 201 has auxiliary line 210, auxiliary line supports 211, and auxiliary line connector 212. Riser joint 201 is attached, as by welding, to auxiliary line support ring 202, which is attached in turn to male element 204. The other end of riser joint 201 is attached to female element 203.
Locking ring 206 is fitted with bayonet-type upper lugs 207A and lower lugs 208A, which interlock cooperatively with upper lugs 207B and lower lugs 208B respectively on female member 203 when locking ring 206 is rotated relative to female member 203. Once the bayonet lugs are properly engaged, locking ring 206 is secured against backing-off (and disengaging the bayonet coupling) by pinned locking ring lock 209. Locking ring 206 is secured in an interlocked position by locking mechanism 209, and the entire riser connection is axially preloaded by tightening ring 205 which, when torqued, bears on shoulder 204A on male element 204. This connector may be run relatively quickly, but due to the complicated load-path and critical tolerances, is difficult to machine and repair, and is relatively heavy and expensive.
Still other prior-art riser connections used radially-displaced shear elements in the box connector to radially grip a profile in the mating pin connector, for example, the pipe connectors taught in U.S. Pat. No. 3,827,728 (“the '728 patent”) issued to Hynes. A later version of this connector taught in U.S Publication No. 2008-0175672 (“the '672 publication”) issued to Fraser uses two staggered rows of radially-displaced shear elements or “dogs.” Like the bayonet riser connections, radial dog riser connections require a certain amount of “supporting metal” in the box riser connection for each dog, with the result that these riser connections tend to be bulky, heavy and expensive.
FIG. 3A illustrates the riser connector as taught in the '728 patent. Riser joints 300 have auxiliary line 301 with auxiliary line connector 302, pin member 303, and box member 304. Auxiliary line 301 is affixed to pin member 303 by integral flange 308, and to the box member 304 by flange 309. Box member 304 has a plurality of windows 305A containing dogs 305 which are arranged to be shifted inwardly to engage one or more circumferentially continuous external grooves 306 in pin member 303. Dogs 305 are substantially arcuate to better match external grooves 306. Windows 305A are circumferentially spaced from one another, generally on a single horizontal plane. Dogs 305 are locked in engagement with external grooves 306 by locking mechanism 309. However, the strength of this riser connection may be limited by the shear area available in the dogs, which in turn is limited by the amount of supporting metal required for each dog and its window. Additionally, the need for a light riser of a relatively small diameter is difficult to achieve due to the limited shear area available.
FIG. 3B illustrates the related riser connector as taught in the '732 publication. This riser riser connection is an attempt to increase the number of load-carrying dogs available in a dog-type riser without increasing the diameter of the connector and with only a minimal increase in weight. Riser 310 has pin member 311 and box member 312. Box member has windows 313A containing dogs 313 and locking mechanisms 314. However, in this connector, windows are disposed in a staggered pattern in a plurality of rows, shown here as upper row 315 and lower row 316; that is, the windows in upper row 315 are circumferentially displaced (or “staggered”) in relationship to the circumferential position of the windows in lower row 316. However, neither the '728 patent of the '732 publication contemplate a redundant load path (e.g., to be used if the primary load is damaged), nor do they contemplate reconditioning of the riser connection on-board a drilling vessel.
Accordingly, there exists a need for a quick-tripping riser connection for a marine drilling riser which is also relatively light in weight, economical to produce, reliable in service, with provision for a redundant or secondary or emergency load-path, which has the capacity for a very high axial preload, and which may be reconditioned cheaply and quickly, even on-board a drilling vessel.